1. Field of the Invention
This invention pertains generally to electromagnetic devices, and more particularly to multiferroic electromagnetic devices.
2. Description of Related Art
Electromagnetic devices, including antennas, motors, and memory, generally rely on extrinsic coupling produced by passing an electrical current through a wire to generate a magnetic field. While extremely successful in the large scale, this approach suffers from significant problems in the small scale where resistive losses are preventing further device miniaturization. An intrinsic approach has been sought to electrically control magnetization, and some minor progress has been made using electric field induced strain to modulate magnetization in multiferroic composite materials. However, these “bulk” multiferroic materials contain multi-domain magnetic structures that produce marginal magnetization changes with the application of an electric field. Recent developments have focused on nanoscale elements, using electric field induced strain to control a single magnetic domain. To date, however, only domain reorientation (i.e. electric fields only reorient the magnetization state) has been achieved and researchers have not been able to use magnetoelectric coupling to control the overall magnetic state of the material (i.e. change its magnitude to turn on or off net magnetization).
One roadblock to achieving miniaturization of magnetic devices is superparamagnetism, and so efforts have been made to control this transition. As the size of magnetic materials decreases, ambient thermal energy becomes higher than intrinsic magnetic anisotropies, resulting in randomization of magnetic orientations and no net time averaged magnetization. Attempts to modulate superparamagnetism have been made using exchange-biasing to shift the superparamagnetic transition temperature. For memory applications, where transient excursions toward the superparamagnetic limit could reduce write energies, heat assisted magnetic memory is also an option that has been considered. For exchange-bias materials, unfortunately the coupling results in only a shift in transition temperature and not control over the magnetic state of the material.